fix: correctly infer output extension for jupytext command

[hamzashezad]

fixes a bug where extracted metadata information is not used when constructing the jupytext command to convert python.
NOTE: this and #19 are both required to fix issues when opening a Google Colab exported notebook.

Context
I get the following metadata from a notebook exported directly from Google Colab. jupytext doesn't have enough information to use auto:hydrogen when converting:

  "metadata": {
    "colab": {
      "provenance": []
    },
    "kernelspec": {
      "name": "python3",
      "display_name": "Python 3"
    },
    "language_info": {
      "name": "python"
    }

[GCBallesteros]

Can you explain what information is jupytext missing above in order to make use of the auto mode?

[hamzashezad]

jupytext uses the kernelspec.language_info.file_extension (or kernelspec.language, or jupytext.main_language as fallbacks) to determine the output format:
https://github.com/mwouts/jupytext/blob/main/src/jupytext/formats.py#L811 -> https://github.com/mwouts/jupytext/blob/main/src/jupytext/formats.py#L782